The present invention relates to composite materials that may be shaped to form a wide variety of prostheses for orthopedic applications. Specifically, compositions that are composites of a biocompatible polymer and a particulate ceramic are provided, wherein the particulate ceramic has an average particle size of not more than about 500 nm.
The intervertebral disc functions to stabilize the spine and to distribute forces between vertebral bodies. A normal disc includes a gelatinous nucleus pulposus, an annulus fibrosis and two vertebral end plates. The nucleus pulposus is surrounded and confined by the annulus fibrosis.
Intervertebral discs may be displaced or damaged due to trauma or disease. Disruption of the annulus fibrosis allows the nucleus pulposus to protrude into the spinal canal, a condition commonly referred to as a herniated or ruptured disc. The extruded nucleus pulposus may press on the spinal nerve, which may result in nerve damage, pain, numbness, muscle weakness and paralysis. Intervertebral discs may also deteriorate due to the normal aging process. As a disc dehydrates and hardens, the disc space height will be reduced, leading to instability of the spine, decreased mobility and pain.
One way to relieve the symptoms of these conditions is by surgical removal of a portion or all of the intervertebral disc. The removal of the damaged or unhealthy disc may allow the disc space to collapse, which could lead to instability of the spine, abnormal joint mechanics, nerve damage, as well as severe pain. Therefore, after removal of the disc, adjacent vertebrae are typically fused to preserve the disc space.
Several devices exist to fill an intervertebral space following removal of all or part of the intervertebral disc in order to prevent disc space collapse and to promote fusion of adjacent vertebrae surrounding the disc space. Many of the implants, including intervertebral disc spacers, as well as cervical plates along with bone screws utilized to hold the plates in place, have been made of metal. Due to the stiffness of the material, some metal implants, such as intervertebral disc spacers, may stress shield the bone, thereby increasing the time required for fusion. Subsidence, or sinking of the device into bone, may also occur when metal implants are implanted between vertebrae if fusion is delayed.
In order to solve the problems associated with metal implants, a wide variety of bone grafts and substitutes thereof have been used. Autograft is often preferred because it is osteoinductive, although allograft may also be used. However, such sources of grafts provide several disadvantages. Autograft is available in only limited supply. Moreover, the additional surgery increases the risk of infection and blood loss and may reduce structural integrity at the donor site. Additionally, some patients complain that the graft harvesting surgery causes more short-term and long-term pain than the fusion surgery.
Allograft material, which is obtained from donors of the same species, is more readily obtained. However, allogeneic bone does not have the osteoinductive potential of autogenous bone and may thus provide only temporary support. The slow rate of fusion using allografted bone can lead to collapse of the disc space before fusion is accomplished.
Several attempts have been made to develop a material that may be used to form an implant, such as an intervertebral disc implant or other load bearing structure, which avoids the disadvantages of metal implants and bone implants. Calcium phosphate-based implants are known, but are brittle in many cases and may thus not have sufficient load bearing capacity for a desired application. Synthetic polymers are also known, but have several drawbacks, including their small load bearing capacity and their minimal or non-existent bone bonding capacity. Although compositions of synthetic polymers and hydroxapatite, typically sized in the micrometer range, have been made in order to combine the advantages of the polymers and the calcium phosphate-based ceramics, and have obtained a certain degree of success, a need for improved compositions for orthopedic use still exists. The present invention addresses this need.